This invention relates to semiconductor light-emitting devices, and more particularly, concerns semiconductor lasers of stripe type.
Hitherto, various kinds of stripe type lasers have been proposed. The stripe type laser has merits not only of decreasing its threshold current, but also of achieving simple lasing mode, resulting in easy use in light-communication.
FIG. 1 shows an example of conventional oxide stripe type laser which is a typical example of the stripe type laser. In FIG. 1, reference numerals designate as follows:
5--n-GaAs (substrate) PA1 1--n-Ga.sub.0.7 Al.sub.0.3 As PA1 2--p-GaAs (active layer) PA1 3--p-Ga.sub.0.7 Al.sub.0.3 As PA1 4--p-GaAs PA1 8--SiO.sub.2 insulation film PA1 6--cathode electrode (Au-Ge alloy film) PA1 7--anode electrode (Au-Sn alloy film) PA1 71--stripe part (contacting part) of the electrode 7 having width "w". The conventional stripe type laser of FIG. 1 has the following shortcomings: The current flowing from the contacting part 71 of the electrode 7 into the semiconductor wafer disperses as shown by the dotted arrows in FIG. 1, and therefore, in the p-GaAs active region 2, the current disperses in the wide area 21 indicated by hatching. Therefore, even though the width of the stripe part 71 of the electrode 7 is limited narrow, width of the effective active region becomes wide, hence making the threshold current large. In such conventional device, also it has been found that the minimum threshold current is obtained when the width w of the contacting part 71 is about 10.mu., and for the w smaller than 10.mu. the threshold current increases instead. The section of actual lasing region (hatched part 21 of the active layer 1 in FIG. 1) of the conventional device as seen in the elevation view (FIG. 1) was of oval shape with the major axis of about 10.mu. and the minor axis of about 0.5.mu., and accordingly, it was necessary to use cylindrical lens in order to lead the light from the active layer 2 into light-conduit glassfiber (not shown). PA1 5--n-GaAs substrate PA1 1--n-Ga.sub.0.7 Al.sub.0.3 As PA1 2--p-GaAs (active layer) PA1 3--p-Ga.sub.0.7 Al.sub.0.3 As PA1 4--p-GaAs PA1 6--cathode electrode (Au-Ge alloy film) PA1 7--anode electrode (Au-Sn alloy film) PA1 9,9--Ga.sub.0.7 Al.sub.0.3 As (of very low impurity concentration).
In order to eliminate the abovementioned shortcomings, another buried stripe-type heterostructure semiconductor laser shown in FIG. 2 has been proposed.
In FIG. 2, reference numerals designate as follows:
As shown in FIG. 2, a certain depth from the surface of the substrate 5, the n-Ga.sub.0.7 Al.sub.0.3 As layer 1, the p-GaAs active layer 2, p-Ga.sub.0.7 Al.sub.0.3 As layer 3 and the p-GaAs layer 4 are mesaetched away at its both side parts so as to retain central stripe part, and the low-impurity concentration Ga.sub.0.7 Al.sub.0.3 As layers 9,9 are formed by liquid phase epitaxial growth in place of etched-away parts. In the conventional device of FIG. 2, the width of the actual active region can be limited to the width w of the stripe part, and hence can be controlled to be equal to the thickness (which is about 1.mu.) of the active layer 2. Therefore, the section of actual lasing region can be made round, and hence the lased light can be easily led into a light-conduit fiber without use of a cylindrical lens. Therefore, the matching between the active region and the light-conduit fiber has been improved. Moreover, since the width w of the layers 4, 3, 2, 1 and the protruding part 51 of the substrate 5 are clearly limited to a predetermined value, and therefore, no dispersion of injection current takes place, thereby enabling lasing with such low current as 10 mA. However, the device of FIG. 2 is very difficult in the manufacture, since due to spontaneously formed oxidized films, on the mesa-etched side-surfaces of the layers 1 and 3, (the oxidized film being likely formed when the layers contain Al), the low impurity concentration region 9 can not regularly adhere on these etched side-surfaces, and since the components of the GaAs regions 2 and 4 are likely to melt in the regions 9,9 thereby changing the width of the active layer 2 from the predetermined designed width. Furthermore, it is very difficult to obtain flat surface of the wafer by stopping the growth of the layers 9,9 at an appropriate timing in order to make the upper faces of the low impurity concentration layers 9,9 flush with that of the stripe shaped layer 4. Besides, the p-GaAs active layer 2 and the Ga.sub.0.7 Al.sub.0.3 As layers 9,9 have difference by about 26% in thermal expansion coefficient from each other, and therefore, during cooling down from 800.degree. C. for forming the layers 9,9 by liquid phase epitaxial growth, to the room temperature, a considerable strain is made on their interface, resulting in adverse effect on the life of the laser device. Furthermore, the Ga.sub.0.7 Al.sub.0.3 As layers 9,9 have so low a heat conduction coefficient as 1/10 of that of GaAs layer 2, and hence, the heat produced in the active layer 2 can not escape through the layers 9,9, but is forced to escape upward and downward only.